Conventional fuel systems for vehicles with internal combustion engines can include a canister that accumulates fuel vapor from a headspace of a fuel tank. If there is a leak in the fuel tank, the canister, or any other component of the fuel system, fuel vapor could escape through the leak and be released into the atmosphere instead of being accumulated in the canister. Various government regulatory agencies, e.g., the U.S. Environmental Protection Agency and the Air Resources Board of the California Environmental Protection Agency, have promulgated standards related to limiting fuel vapor releases into the atmosphere. Thus, it is believed that there is a need to avoid releasing fuel vapors into the atmosphere, and to provide an apparatus and a method for performing a leak diagnostic, so as to comply with these standards. Emission standards also stipulate that the performance of each emission control device be monitored (e.g., a canister purge valve).
In such conventional fuel systems, excess fuel vapor can accumulate immediately after engine shutdown, thereby creating a positive pressure in the fuel vapor pressure management system. Excess negative pressure in closed fuel systems can occur under some operating and atmospheric conditions, thereby causing stress on components of these fuel systems. Thus, it is believed that there is a need to vent, or “blow-off,” the positive pressure, and to vent, or “relieve,” the excess negative pressure. Similarly, it is also believed to be desirable to relieve excess positive pressure that can occur during tank refueling. Thus, it is believed that there is a need to allow air, but not fuel vapor, to exit the tank at high flow rates during tank refueling. This is commonly referred to as onboard refueling vapor recovery (ORVR).
When the engine is not running, excessive fuel vapor is typically stored in a canister that contains charged charcoal for trapping the hydrocarbons. Fuel vapor stored within this canister is recovered when the engine is running by airflow through the canister resulting from the engine intake vacuum. A canister purge valve is located between the canister and engine intake to regulate the amount of fuel vapor drawn into the engine. If there is excess fuel vapor upstream of the canister purge valve, as a possible result of the purge valve regulating the flow of fuel vapor as intended, then excessive vapor can build up and possibly leak into the atmosphere, thereby giving rise to environmental contamination concerns.